It's even simpler than ForceUser@Home is making it out to be . . . it's up to the GM. Simple as that.
Quoting books and using them as exacting standards in situations the authors probably didn't think of is, well, silly. It's like two different flavored Christians quoting the same Bible verse at each other telling the other they're not real Christians.
Just use what makes sense to you. All right, so on page 130 of the PHB, an ally directly opposite you has to threaten your opponent. So? Does that mean that if there's a 3-party combat where a non-"ally" is attacking your opponent, you don't get a bonus because the "flanker" isn't your ally?
That's what it says in the PHB. But I believe, as a GM, you should use common sense, instead. If you're GM and feel illusions can't flank, so be it. But never bother trying to read rules so they fit what you think (90% of this board). If you're playing in the grey, just use your noggin'.